Extension of the sim card in gsm devices

ABSTRACT

Circuit arrangement as an interface between a SIM card and a GSM modem, which has a bidirectional data line that connects a card data input/output of the SIM card to a modem data input/output of the GSM modem, with the data line being coupled to at least one edge driver.

FIELD OF TECHNOLOGY

The present disclosure relates to a circuit arrangement and to a methodfor bidirectional data transmission.

BACKGROUND

SIM cards have become established as the standard for authentication ofa user of a GSM mobile radio. In conventional GSM appliances, such as amobile telephone, the card reader for SIM cards is integrated in theappliance. As the functionality of GSM appliances increases, theimportance of the SIM card is also growing, however, and the SIM card isno longer used just to check the authorization to make telephone calls,but for different applications, such as access authorization to secureareas, that is to say in the end as a key substitute, or for paymentfunctions.

In this case, it has been found that, for some applications, physicalseparation between the SIM card reader and the GSM appliance isdesirable. For example, the SIM card may be inserted in a correspondingcard reader in a motor vehicle, in order to authenticate the user of aGSM system in the vehicle in this way. Furthermore, both in the privatefield and in the commercial field, a user may be authenticated by meansof a SIM card reader for telephone systems, computers, network parts,automatic payment machines or other appliances with GSM functionality.In this case, for security reasons, a direct link may be providedbetween the card reader and the GSM appliance for the transmission ofthe authentication data and, in the applications which have beenmentioned, this must often have a length of several meters, owing to thephysical characteristics.

Generally, however, the electrical drivers for the SIN card interface inGSM modems are designed only for distances of less than 50 cm.

Various currently marketed integrated circuits allow this to be extendedto several meters. However, since the data line to the SIM card isdesigned to be bidirectional, these circuits require a control signalfor the signal direction. However, no such signal is externallyavailable on standard GSM modems.

Some of the abovementioned circuits are thus designed only for directintegration in GSM modems, which has the disadvantage that, as a resultof the modern manufacturing methods, each of the GSM modems that isproduced in a range must be equipped with a circuit such as this,irrespective of whether or not this is desired. A further disadvantageof these circuits is that they are costly.

It is also possible to arrange a circuit arrangement for directionevaluation of the data line outside the GSM modem, and thus to controldata line drivers which may comprise an integrated circuit or otherelectronic components. However, a circuit arrangement such as this has anumber of disadvantages. In addition to the fact that its implementationinvolves a high degree of complexity, the use of simple line driversresults in the problem of the entire SIM interface being switched to aninactive, high-impedance state. Furthermore, a circuit apparatus such asthis with a signal direction evaluator requires additional cable wires,which involves additional complexity in manufacturing, and thusincreased production costs.

Another possibility is to use GSM modems with a remote access functionfor SIM data (so-called remote SIM access or RSA). In this case, thedata is written to and read from the SIM card at a remote point, and isinterchanged between the GSM modem and the SIM card by means of ATcommands (Hayes standard command set for modems, from ATtention). Thedata is in this case transmitted via wire or radio (for exampleBluetooth). This solution is likewise costly and is dependent, moreover,on the use of GSM modems with RSA functionality, which are availableonly in limited numbers on the market.

SUMMARY

An apparatus and method is disclosed to provide the capability toconnect a separate SIM card reader to a standard GSM modem which hasneither an external signal for the data signal direction nor RSAfunctionality. At the same time, the intention is thus to create thebasis for the capability to advantageously lengthen the connecting pathto several meters.

Under an exemplary embodiment, the data line between the two appliancesto be connected, that is to say the GSM modem and the SIM card reader,is coupled to at least one edge driver. The edge driver is in this caseused as a driver for the bidirectional data line and amplifies acorresponding edge of the signal, in order in this way to counteracteffects which result from the capacitance of the connecting line andlead to flattening of the edge. The edge drivers assist the change inthe charge in the capacitance of the extension line, and carry this outquickly. After the change in the charge in the capacitance, the edgedrivers have no further effect, so that there is no further influence onstatic signals. Furthermore, the edge drivers allow the SIM interface tobe lengthened without signal evaluation and without any signal directionsignal, with the functionality of the interface being restricted.

This dynamic response of the edge drivers means that there is noinfluence on the steady-state characteristics of the SIM interface. Insome applications, by way of example, the hardware of the SIM interfacemust be run down (switched off) completely. This switching-off iscarried out by the GSM modem. Since the edge drivers act dynamically,they do not impede this function, thus ensuring unrestricted operationof the interface.

Under the embodiment, the data line is coupled to at least one edgedriver both at the modem end and at the card end. The edge drivers inthis case act on the respective active end, that is to say on the end ofthe data line connected to the output as a driver for the extensionline, and at the passive end, that is to say at the end of the data linewhich is connected to the input, as a signal regenerator.

In an alternate embodiment, positive and negative edge drivers are used.Positive edge drivers are intended for the positive edge of the signal,that is to say for a rising edge; negative edge drivers support negativeedges in a corresponding manner. Simultaneous use of positive andnegative edge drivers ensures optimum conditioning of the data signal.

In a further embodiment, only positive edge drivers are used. Thiscircuit arrangement provides a simple and low-cost embodiment of thepresent invention, with the operational capability of this embodimentbeing restricted by the signal quality which the modem and the SIM cardcan produce and (still) process, and thus also or predominantly by thelength of the extension line.

The edge driver(s) is preferably constructed from discrete components.In particular, this offers a low-cost solution to the object on whichthe invention is based. As an alternative to this, the edge driver(s)may also be in the form of an integrated circuit, which ensures thatlittle space is required.

Under the embodiment, each edge driver may in each case be matched todifferent signal frequencies, in particular by the capacitance of acoupling capacitor which couples the edge driver to the data line. Thisallows a broad range of use with regard to the frequencies that areused.

In a further embodiment, a resistor which is connected downstream fromthe coupling capacitor ensures an improvement in the interferencevoltage separation. This improves the functional reliability of thecircuit arrangement, and improves the quality of the data transmission.

The response threshold of the edge driver(s) may, in a furtheradvantageous embodiment, be set or tuned in particular by the insertionof a resistor into the circuit, with tunability being achieved, forexample, with the aid of a potentiometer or a switchable resistancenetwork.

The circuit arrangement is preferably characterized by a capacitor whichis used to improve the response to transient interference. This alsoimproves the functional reliability of the circuit arrangement accordingto the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The various objects, advantages and novel features of the presentdisclosure will be more readily apprehended from the following DetailedDescription when read in conjunction with the enclosed drawings, inwhich:

FIG. 1 shows a schematic circuit diagram or block diagram of a GSM modemand of a SIM card with a holder, having two connecting lines accordingto an exemplary embodiment;

FIG. 2 shows the circuit diagram of a first embodiment of a positiveedge driver;

FIG. 3 shows the circuit diagram of a first embodiment of a negativeedge driver;

FIG. 4 shows the circuit diagram of a second embodiment of a positiveedge driver; and

FIG. 5 shows the circuit diagram of a second embodiment of a negativeedge driver.

DETAILED DESCRIPTION

FIG. 1 shows a schematic illustration of a SIM card 1 with a holder, anda GSM modem 2. The SIM card 1 and the GSM modem 2 are connected to oneanother via the following lines: a CCVCC line 3, which provides the SIMcard 1 with an operating voltage of 3 volts; a CCGND line 4, whichprovides the SIM card 1 with ground potential; a CCIO line 5, which isused as a bidirectional data line; a CCCLK line 6, which represents theclock line, with normal clock frequencies being approximately 1 MHz toapproximately 4 MHz, and a CCRST line 7 as a reset line for thearrangement. The desired lengthening of the lines which connect the SIMcard 1 to the GSM modem 2 is indicated by the extension line 8 which isillustrated in the center of the figure.

The SIM card holder additionally has a switching contact 9 which on theone hand monitors whether any SIM card 1 at all has been inserted intothe card holder, and on the other hand evaluates whether the SIM card 1is withdrawn from the SIM card holder during operation of thearrangement. In the present exemplary embodiment, the switching contactis designed as a leading contact, that is to say, by monitoring thelevel at a COIN input 10 while the SIM card is being withdrawn from theSIM card holder, the GSM modem has the capability to run down the entireSIM interface and to switch it to a high impedance even before theinterface contacts are interrupted during the withdrawal of the SIM cardfrom the SIM card holder.

The abovementioned lines and the respective lengthenings of the lines aswell as the preconditions for the lengthening of the lines will beconsidered in more detail in the following text: the lengthening of theoperating voltage line (CCVCC line 3) and of the SIM ground line (CCGNDline 4) preferably do not require any particular Requirements. However,a supporting capacitor is preferably provided (not illustrated inFIG. 1) at the card end between the CCVCC line 3 and the CCGND line 4 inorder to decouple the operating voltage from the line conditions at thecard end. Furthermore, the line should have as low an impedance aspossible.

The CCIO line 5 is a bidirectional line which, as already mentioned,represents the data line in the arrangement. This means that both theGSM modem 2 and the SIM card 1 can transmit and receive alternately onthe same line. In order to avoid damage in the event of conflicts, thatis to say if the GSM modem 2 and the SIM card 1 are both connected tothe output and transmit at the same time as one another, the outputs atboth ends are in the form of open drains. The common drain resistor 11which is required for this purpose is integrated in the GSM modem 2.

In order to rapidly change the charge in the capacitance of theextension line or, to be more precise, in order to assist theabovementioned change in the charge, positive edge drivers 12 andnegative edge drivers 13 are coupled to the CCIO line 5 at both themodem end and at the card end, and their design and method of operationwill be explained in more detail later. Once the charge on thecapacitance of the CCIO line 5 has been changed, including that on itsextension, neither the positive edge drivers 12 nor the negative edgedrivers 13 have any effect.

The edge drivers in this case act on the respective active end, that isto say the end which is connected to the output, as a driver for thesignal on the CCIO line 5, including its extension, and act as a signalregenerator at the passive end, that is to say the end which isconnected to the input. The edge of the signal at the transmitting endis thus supported, that is to say the signal which is used to charge thecapacitance of the line is amplified, so that the edge is kept as steepas possible; at the receiving end, the edge of the signal isadditionally conditioned once again in a further amplification step.

At this point, it should be noted that there is no absolute requirementfor four edge drivers, depending on the line length and the signalquality produced at the respective CCIO output 14. In this case, it isalso possible to use only one positive edge driver 12 or, if the linelengths are relatively large, two positive edge drivers 12 without anynegative edge driver 13 and, alternatively, a combination of positiveedge drivers 12 and negative edge drivers 13.

The CCCLK line 6 is a unidirectional line from the GSM modem 2 to theSIM card 1. The clock driver stage (not illustrated in FIG. 1) which isintegrated in the GSM modem 2 has a so-called push-pull output. In orderto reduce the amount of the emitted radiation, the clock line islengthened in a balanced form. An inverter 15 is provided for thispurpose, which can be tuned by means of a resistor 16 to have the sameinternal resistance as the clock line (CCCLK line 6). Furthermore,resistors 17, 18 are provided as a line termination at the SIM card endfor this purpose, and are connected to the SIM card ground CCGND 4,without any direct current, via capacitors 19, 20.

It should be noted here that if the lengths of the extension line arelong, edge drivers can also be coupled to the CCCLK line 6. It shouldalso be noted that the abovementioned method of balanced extension ofthe line can also be applied to other lines, such as the CCIO line 5.

The CCRST line 7 has a unidirectional signal flow from the GSM modem 2to the SIM card 1. The driver stage, which is integrated in the GSMmodem 2, has a push-pull output. It should be noted here, too, that,particularly in the case where the extension lines are long and when thedriver capability of the push-pull output is poor, one or more edgedrivers may be coupled to the CCRST line 7.

The position of the switching contact 9 on the card holder is signaledat the CCIN input 10. If the contact is used to run down the SIMinterface, rapid transmission of the information is necessary. A driverwhich comprises a transistor 21 and two resistors 22, 23 is thereforeprovided at the card end. If the SIM card is withdrawn from the SIM cardholder during operation, then, by monitoring the level at the CCIN input10, it is possible to run down the entire SIM interface, and to switchit to a high impedance, even before the interface contacts areinterrupted during the withdrawal of the SIM card.

The following text describes the method of operation of a positive edgedriver 12 and of a negative edge driver 13 in more detail on the basisof FIGS. 2 and 3. The edge drivers are formed from discrete components;the positive edge driver and the negative edge driver are complementaryto one another, but in principle are designed identically. Theiroperation will be explained with reference to a positive edge driver,first of all, in the following text:

It is assumed that the signal on the CCIO line 5 is changing from thepotential of the CCGND line 4 to that of the CCVCC line 3. When there isa positive change to the line level of CCIO line 5 as mentioned above, atransistor 24 is switched on as soon as its base-emitter thresholdvoltage (approximately 0.6V) is exceeded, with the line level of theCCIO line 5 being input via a capacitor 25.

When the transistor 24 is switched on, it switches a transistor 28 onvia a voltage divider which is formed from resistors 26, 27. Via itscollector, the transistor 28 raises the CCIO line 5 to the positivepotential of the CCVCC line 3. This potential increase in turn acts onthe base of the transistor 24, via the capacitor 25. This is thereforean amplifier arrangement with dynamic positive feedback. The describedmechanism is active only for as long as the capacitor 25 is charged.

The important factor in this case is that the capacitor 25 has acapacitance such that the charge on the capacitor 25 can always bechanged completely (charging and discharging) between two edges of thesignal, depending on the signal frequency. The discharging of thecapacitor 25 is assisted by means of a diode 29 when a negative edgeoccurs. Once the capacitor 25 has been charged, the entire circuitarrangement has no more effect until the next positive signal edge. Thisensures that it is tolerant to steady-state signal levels and to the SIMinterface being run down electrically by the GSM modem 2.

A resistor 30 is connected downstream from the capacitor 25 in order toimprove the signal-to-noise ratio. In conjunction with a resistor 31,the response threshold of the transistor 24 is raised, thus increasingthe signal-to-noise ratio.

The operation of the negative edge driver is explained as follows: whenthe potential on the CCIO line 5 changes from the potential on the CCVCCline 3 to that on the CCGND line 4, then a transistor 24′ is switched onas soon as its base-emitter threshold voltage (approximately 0.6V) isexceeded. The line level is input in an analogous manner to that for thepositive edge driver 12 via a capacitor 25′. The switched-on transistor24′ switches a transistor 28′ on via a voltage divider which is formedfrom resistors 26′, 25 27′, and via its collector, this transistor 28′reduces the potential on the CCIO line 5 to the ground potential on theCCGND line 4. This reduction in turn acts on the base of the transistor24′ via the capacitor 25′. Once again, this is an amplifier arrangementwith dynamic positive feedback. The described mechanism is active onlyfor as long as the capacitor 25′ is charged.

The design requirements relating to the capacitance of the capacitor 25′are the same as those for the positive edge driver. The discharging ofthe capacitor 25′ when a positive edge occurs is assisted by a diode29′. Once the capacitor 25′ has been charged, the entire circuitarrangement has no more effect until the next negative signal edge. Onceagain, the circuit arrangement is in consequence tolerant tosteady-state signal levels and to electrical running down of the SIMinterface by the GSM modem 2.

Analogously to the positive edge driver 12, a resistor 30′ may also beconnected downstream from the capacitor 25′ in the case of the negativeedge driver 13, in order to improve the signal-to-noise ratio. Inconjunction with a resistor 31′, the response threshold of thetransistor 24′ is in this case increased.

A positive edge driver 12 which has a lower response threshold ratherthan an improved interference separation (as shown in FIG. 2) isillustrated in FIG. 4. The corresponding negative edge driver is shownin FIG. 5. A resistor 32 which, together with the resistor 31, forms avoltage divider is connected downstream from the capacitor, instead ofthe resistor 30. In order to improve the response to transientinterference, a capacitor 33 is also connected in parallel with thebase-emitter junction of the transistor 28. This leads to a delay in theresponse time of the transistor 28.

Analogously to the positive edge driver shown in FIG. 4, the negativeedge driver shown in FIG. 5 has a resistor 32′ and a capacitor 33′,which carry out the same function as the resistor 32 and the capacitor33.

While the invention has been described with reference to one or moreexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiments disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1-14. (canceled)
 15. An interface between a SIM card and a GSM modemcomprising: a bidirectional data line that connects a card datainput/output of the SIM card to a modem data input/output of the GSMmodem, wherein the data line is coupled to at least one edge driver. 16.The interface as claimed in claim 15, wherein the data line is coupledto at least one edge driver, both at the modem end and at the card end.17. The interface as claimed in claim 15, wherein that positive andnegative edge drivers are provided.
 18. The interface as claimed inclaim 15, wherein that only positive edge drivers are provided.
 19. Theinterface as claimed in claim 1, wherein that the at least one edgedriver is formed from discrete components.
 20. The interface as claimedin claim 5, wherein that the at least one edge driver is in each casematched to different signal frequencies, in particular by thecapacitance of a coupling capacitor which couples the edge drivers tothe data line.
 21. The interface as claimed in claim 20, wherein aresistor is connected downstream from the coupling capacitor, in orderto improve the interference voltage separation.
 22. The interface asclaimed in claim 21, wherein the response threshold of the or each edgedriver is set or tuned in by a second resistor coupled to the edgedriver.
 23. The interface as claimed in claim 22, wherein a secondcapacitor coupled to the edge driver in order to improve the response totransient interference.
 24. A method for bidirectional data transmissionbetween a SIM card and a GSM modem wherein the bidirectional datatransmission takes place without the use of a control signal for thedata direction on a data line that connects the SIM card and the GSMmodem.
 25. The method as claimed in claim 24, wherein at least one edgedriver is used for conditioning of the signal on the data line.
 26. Themethod as claimed in claim 25, wherein the at least one edge driver canin each case be optimized to the clock rate of the data transmission byinserting a coupling capacitor.
 27. The method as claimed in claim 25,wherein the interference voltage separation of the at least one edgedriver can in each case be set, by means of a resistor.
 28. The methodas claimed in claim 25, wherein the response threshold of the at leastone edge driver can in each case be set or tuned, by means of aresistor.